Methods and apparatus for spatial light modulation

ABSTRACT

Improved apparatus and methods for spatial light modulation are disclosed which utilize optical cavities having both front and rear reflective surfaces. Light-transmissive regions are formed in the front reflective surface for spatially modulating light.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Utility patent application Ser.No. 11/594,042 filed on Nov. 6, 2006, entitled “Methods and Apparatusfor Spatial Light Modulation”, which is a continuation of U.S. Utilitypatent application Ser. No. 11/218,690 filed on Sep. 2, 2005, whichclaims the priority to and benefit of Provisional Patent Application No.60/676,053, entitled “MEMS Based Optical Display” and filed on Apr. 29,2005, and U.S. Provisional Patent Application No. 60/655,827, entitled“MEMS Based Optical Display Modules” and filed on Feb. 23, 2005. Theentirety of each of these applications is incorporated herein byreference.

FIELD OF THE INVENTION

In general, the invention relates to the field of spatial lightmodulation, in particular, the invention relates to displays havingimproved backlights.

BACKGROUND OF THE INVENTION

Displays built from mechanical light modulators are an attractivealternative to displays based on liquid crystal technology. Mechanicallight modulators are fast enough to display video content with goodviewing angles and with a wide range of color and grey scale. Mechanicallight modulators have been successful in projection displayapplications. Backlit displays using mechanical light modulators havenot yet demonstrated sufficiently attractive combinations of brightnessand low power. When operated in transmissive mode many mechanical lightmodulators, with aperture ratios in the range of 10 and 20%, are onlycapable of delivering 10 to 20% of available light from the backlight tothe viewer for the production of an image. Combining the mechanicalapertures with color filters reduces the optical efficiency to about 5%,i.e., no better than the efficiencies available in current color liquidcrystal displays. There is a need for a low-powered display havingincreased luminous efficiency.

SUMMARY OF THE INVENTION

The devices and methods described herein provide for mechanical lightmodulators having improved luminous efficiency, making mechanicalactuators attractive for use in portable and large area displays. Insome cases, the transmittance or optical efficiency of mechanicalmodulators coupled to backlights can be improved to the 40 to 60% level,or 10 times more efficient than what is typical in a liquid crystaldisplay. In addition, the devices and methods described herein can beincorporated into small-size, high resolution displays, regardless ofthe light modulation mechanism, to improve the brightness of thedisplays and to reduce the power requirements in a display application.

The light modulators described herein make possible portable videodisplays that can be both bright and low power. The light modulators canbe switched fast enough to provide color images using time sequentialcolor techniques, instead of relying on color filters. The displays canbe built using as few as three functional layers to form both amechanical shutter assembly and the electrical connections necessary forarray addressing.

In one aspect, the invention relates to a display apparatus thatincludes a layer of reflective material and an array of MEMS-based lightmodulators supported proximate the layer of reflective material. Each ofthe MEMS-based light modulators includes a modulation element and anactuator for moving the modulation element relative to the layer ofreflective material. In one embodiment, the reflective material includesapertures for allowing light to pass through the reflective layer. Eachaperture corresponds to one of the MEMS based light modulators.

BRIEF DESCRIPTION OF THE FIGURES

The system and methods may be better understood from the followingillustrative description with reference to the following drawings inwhich:

FIG. 1A is an isometric conceptual view of an array of light modulators,according to an illustrative embodiment of the invention;

FIG. 1B is a cross-sectional view of a shutter assembly included in thearray of light modulators of FIG. 1A, according to an illustrativeembodiment of the invention;

FIG. 1C is an isometric view of the shutter layer of the shutterassembly of FIG. 1B, according to an illustrative embodiment of theinvention;

FIG. 1D is a top view of the various functional layers of a lightmodulation array, such as the light modulation array of FIG. 1A;

FIG. 2 is a cross-sectional view of an optical cavity for use in aspatial light modulator, according to an illustrative embodiment of theinvention;

FIGS. 3A-3D are cross-sectional views of alternative shutter assemblydesigns, according to illustrative embodiments of the invention;

FIG. 4 is a cross-sectional view of a shutter assembly having a firstcoated shutter, according to an illustrative embodiment of theinvention;

FIG. 5 is a cross-sectional view of a shutter assembly having a secondcoated shutter, according to an illustrative embodiment of theinvention;

FIG. 6 is a cross-sectional view of a shutter assembly having an elasticactuator for use in the light modulation array, according to anillustrative embodiment of the invention;

FIG. 7 is a cross-sectional view of a shutter assembly having adeforming shutter for use in the light modulation array, according to anillustrative embodiment of the invention;

FIGS. 8A-8B are cross-sectional views of the shutter assemblies built onopaque substrates for use in the light modulation array, according to anillustrative embodiment of the invention;

FIG. 9 is a cross-sectional view of a liquid crystal-based spatial lightmodulator, according to an illustrative embodiment of the invention;

FIG. 10 is a cross-sectional view of a first shutter-based spatial lightmodulator, according to an illustrative embodiment of the invention;

FIG. 11 is a cross-sectional view of a second shutter-based spatiallight modulator, according to the illustrative embodiment of theinvention;

FIGS. 12A-12D are cross-sectional views of third, fourth, fifth, andsixth illustrative shutter-based spatial light modulators, according toan embodiments of the invention;

FIG. 13 is a cross-sectional view of a seventh shutter-based spatiallight modulator, according to an illustrative embodiment of theinvention;

FIGS. 14A and 14B are cross-sectional views of two additional spatiallight modulators, according to an illustrative embodiment of theinvention;

FIG. 15 is a cross-sectional view of an additional shutter assembly,according to an illustrative embodiment of the invention;

FIG. 16 is a cross-sectional view of still a further spatial lightmodulator, according to an illustrative embodiment of the invention;

FIG. 17 is an illustrative transflective shutter assembly, according toan embodiment of the invention;

FIG. 18 is a second illustrative transflective shutter assembly,according to an embodiment of the invention;

FIG. 19 is a cross-sectional view of a front reflective shutterassembly, according to an illustrative embodiment of the invention; and

FIG. 20 is an isometric view of a larger scale display formed from anarray of light modulation arrays, according to an illustrativeembodiment of the invention.

DESCRIPTION OF CERTAIN ILLUSTRATIVE EMBODIMENTS

To provide an overall understanding of the invention, certainillustrative embodiments will now be described, including apparatus andmethods for spatially modulating light. However, it will be understoodby one of ordinary skill in the art that the systems and methodsdescribed herein may be adapted and modified as is appropriate for theapplication being addressed and that the systems and methods describedherein may be employed in other suitable applications, and that suchother additions and modifications will not depart from the scope hereof.

FIG. 1A is an isometric conceptual view of an array 100 of lightmodulators (also referred to as a “light modulation array 100”),according to an illustrative embodiment of the invention. The lightmodulation array 100 includes a plurality of shutter assemblies 102a-102 d (generally “shutter assemblies 102”) arranged in rows andcolumns. In general, a shutter assembly 102 has two states, open andclosed (although partial openings can be employed to impart grey scale).Shutter assemblies 102 a and 102 d are in the open state, allowing lightto pass. Shutter assemblies 102 b and 102 c are in the closed state,obstructing the passage of light. By selectively setting the states ofthe shutter assemblies 102 a-102 d, the light modulation array 100 canbe utilized to form an image 104 for a projection or backlit display,illuminated by lamp 105. In the light modulation array 100, each shutterassembly corresponds to a pixel 106 in the image 104. In alternativeimplementations, a light modulation array includes three color-specificshutter assemblies for each pixel. By selectively opening one or more ofthe color-specific shutter assemblies corresponding to the pixel, theshutter assembly can generate a color pixel in the image.

The state of each shutter assembly 102 can be controlled using a passivematrix addressing scheme. Each shutter assembly 102 is controlled by acolumn electrode 108 and two row electrodes 110 a (a “row openelectrode”) and 110 b (a “row close electrode”). In the light modulationarray 100, all shutter assemblies 102 in a given column share a singlecolumn electrode 108. All shutter assemblies in a row share a common rowopen electrode 110 a and a common row close electrode 110 b. An activematrix addressing scheme is also possible. Active matrix addressing (inwhich pixel and switching voltages are controlled by means of a thinfilm transistor array) is useful in situations in which the appliedvoltage must be maintained in a stable fashion throughout the period ofa video frame. An implementation with active matrix addressing can beconstructed with only one row electrode per pixel.

In the passive matrix addressing scheme, to change the state of ashutter assembly 102 from a closed state to an open state, i.e., to openthe shutter assembly 102, the light modulation array 100 applies apotential to the column electrode 108 corresponding to the column of thelight modulation array 100 in which the shutter assembly 102 is locatedand applies a second potential, in some cases having an oppositepolarity, to the row open electrode 110 a corresponding to the row inthe light modulation array 100 in which the shutter assembly 102 islocated. To change the state of a shutter assembly 102 from an openstate to a closed state, i.e., to close the shutter assembly 102, thelight modulation array 100 applies a potential to the column electrode108 corresponding to the column of the light modulation array 100 inwhich the shutter assembly 102 is located and applies a secondpotential, in some cases having an opposite polarity, to the row closeelectrode 110 b corresponding to the row in the light modulation array100 in which the shutter assembly 102 is located. In one implementation,a shutter assembly changes state in response to the difference inpotential applied to the column electrode and one of the row electrodes110 a or 110 b exceeding a predetermined switching threshold.

To form an image, in one implementation, light modulation array 100 setsthe state of each shutter assembly 102 one row at a time in sequentialorder. For a given row, the light modulation array 100 first closes eachshutter assembly 102 in the row by applying a potential to thecorresponding row close electrode 110 b and a pulse of potential to allof the column electrodes 108. Then, the light modulation array 100 opensthe shutter assemblies 102 through which light is to pass by applying apotential to the row open electrode 110 a and applying a potential tothe column electrodes 108 for the columns which include shutterassemblies in the row which are to be opened. In one alternative mode ofoperation, instead of closing each row of shutter assemblies 102sequentially, after all rows in the light modulation array 100 are setto the proper position to form an image 104, the light modulation array100 globally resets all shutter assemblies 102 at the same time byapplying a potentials to all row close electrodes 110 b and all columnelectrodes 108 concurrently. In another alternative mode of operation,the light modulation array 100 forgoes resetting the shutter assemblies102 and only alters the states of shutter assemblies 102 that need tochange state to display a subsequent image 104.

In addition to the column electrode 108 and the row electrodes 110 a and110 b, each shutter assembly includes a shutter 112 and an aperture 114.To illuminate a pixel 106 in the image 104, the shutter is positionedsuch that it allows light to pass, without any significant obstruction,through, the aperture 114 towards a viewer. To keep a pixel unlit, theshutter 112 is positioned such that it obstructs the passage of lightthrough the aperture 114. The aperture 114 is defined by an area etchedthrough a reflective material in each shutter assembly, such as thecolumn electrode 108. The aperture 114 may be filled with a dielectricmaterial.

FIG. 1B is a cross sectional diagram (see line A-A′ below in FIG. 1D) ofone of the shutter assemblies 102 of FIG. 1A, illustrating additionalfeatures of the shutter assemblies 102. Referring to FIGS. 1A and 1B,the shutter assembly 102 is built on a substrate 116 which is sharedwith other shutter assemblies 102 of the light modulation array 100. Thesubstrate 116 may support as many as 4,000,000 shutter assemblies,arranged in up to about 2000 rows and up to about 2000 columns.

As described above, the shutter assembly 102 includes a column electrode108, a row open electrode 110 a, a row close electrode 110 b, a shutter112, and an aperture 114. The column electrode 108 is formed from asubstantially continuous layer of reflective metal, the column metallayer 118, deposited on the substrate 116. The column metal layer 118serves as the column electrode 108 for a column of shutter assemblies102 in the light modulation array 100. The continuity of the columnmetal layer 118 is broken to electrically isolate one column electrode108 from the column electrodes 108 of shutter assemblies 102 in othercolumns of the light modulation array 100. As mentioned above, eachshutter assembly 102 includes an aperture 114 etched through the columnmetal layer 118 to form a light-transmissive region.

The shutter assembly includes a row metal layer 120, separated from thecolumn metal layer 118 by one or more intervening layers of dielectricmaterial or metal. The row metal layer 120 forms the two row electrodes110 a and 110 b shared by a row of shutter assemblies 102 in lightmodulation array 100. The row metal layer 120 also serves to reflectlight passing through gaps in the column metal layer 118 other than overthe apertures 114. The column metal layer and the row metal layer arebetween about 0.1 and about 2 microns thick. In alternativeimplementations, such as depicted in FIG. 1D (described below), the rowmetal layer 120 can be located below the column metal layer 118 in theshutter assembly 102.

The shutter 102 assembly includes a third functional layer, referred toas the shutter layer 122, which includes the shutter 112. The shutterlayer 122 can be formed from metal or a semiconductor. Metal orsemiconductor vias 124 electrically connect the column metal layer 118and the row electrodes 110 a and 110 b of the row metal layer 120 tofeatures on the shutter layer 122. The shutter layer 122 is separatedfrom the row metal layer 120 by a lubricant, vacuum or air, providingthe shutter 112 freedom of movement.

FIG. 1C is a isometric view of a shutter layer 122, according to anillustrative embodiment of the invention. Referring to both FIGS. 1B and1C, the shutter layer 122, in addition to the shutter 112, includes fourshutter anchors 126, two row anchors 128 a and 128 b, and two actuators130 a and 130 b, each consisting of two opposing compliant beams. Theshutter 112 includes an obstructing portion 132 and, optionally, asdepicted in FIG. 1C, a shutter aperture 134. In the open state, theshutter 112 is either clear of the aperture 114, or the shutter aperture134 is positioned over the aperture 134, thereby allowing light to passthrough the shutter assembly 102. In the closed state, the obstructingportion 132 is positioned over the aperture, obstructing the passage oflight through the shutter assembly 102. In alternative implementations,a shutter assembly 102 can include additional apertures 114 and theshutter 112 can include multiple shutter apertures 134. For instance, ashutter 112 can be designed with a series of narrow slotted shutterapertures 134 wherein the total area of the shutter apertures 134 isequivalent to the area of the single shutter aperture 134 depicted inFIG. 1C. In such implementations, the movement required of the shutterto move between open and closed states can be significantly reduced.

Each actuator 130 a and 130 b is formed from two opposing compliantbeams. A first pair of compliant beams, shutter actuator beams 135,physically and electrically connects each end of the shutter 112 to theshutter anchors 126, located in each corner of the shutter assembly 102.The shutter anchors 126, in turn, are electrically connected to thecolumn metal layer 118. The second pair of compliant beams, row actuatorbeams 136 a and 136 b extends from each row anchor 128 a and 128 b. Therow anchor 128 a is electrically connected by a via to the row openelectrode 110 a. The row anchor 128 b is electrically connected by a viato the row close electrode 110 b. The shutter actuator beams 135 and therow actuator beams 136 a and 136 b (collectively the “actuator beams 135and 136”) are formed from a deposited metal, such as Au, Cr or Ni, or adeposited semiconductor, such as polycrystalline silicon, or amorphoussilicon, or from single crystal silicon if formed on top of a buriedoxide (also known as silicon on insulator). The actuator beams 135 and136 are patterned to dimensions of about 1 to about 20 microns in width,such that the actuator beams 135 and 136 are compliant.

FIG. 1D is a top-view of the various functional layers of a lightmodulation array 100′, according to an illustrative embodiment of theinvention. The light modulation array 100′ includes twelve shutterassemblies 102′a-102′1, in various stages of completion. Shutterassemblies 102′a and 102′b include just the column metal layer 118′ ofthe light modulation array 100′. Shutter assemblies 102′c-102′f includejust the row metal layer 120′ (i.e., the row open electrode and therow-close electrode) of the light modulation array 100′. Shutterassemblies 102′g and 102′h include the column metal layer 118′ and therow metal layer 120′. In contrast to the shutter assembly 102 in FIG.1B, the column metal layer 118′ is deposited on top of the row metallayer 120′. Shutter assemblies 102′i-1 depict all three functionallayers of the shutter assemblies 102′, the row metal layer 120′, thecolumn metal layer 118′, and a shutter metal layer 122′. The shutterassemblies 102′i and 102′k are closed, indicated by the column metallayer 118′ being visible through the shutter aperture 134′ included inthe shutter assemblies 102′i and 102′k. The shutter assemblies 102′j and102′1 are in the open position, indicated by the aperture 114′ in thecolumn metal layer 118′ being visible in the shutter aperture 134′.

In other alternate implementations, a shutter assembly can includemultiple apertures and corresponding shutters and actuators (forexample, between, 1 and 10) per pixel. In changing the state of thisshutter assembly, the number of actuators activated can depend on theswitching voltage that is applied or on the particular combination ofrow and column electrodes that are chosen for receipt of a switchingvoltage. Implementations are also possible in which partial openings ofan aperture is made possible in an analog fashion by providing aswitching voltages partway between a minimum and a maximum switchingvoltage. These alternative implementations provide an improved means ofgenerating a grey scale.

With respect to actuation of shutter assemblies 102, in response toapplying a potential to the column electrode 108 of the shutter assembly102, the shutter anchors 126, the shutter 112 and the shutter actuatorbeams 135 become likewise energized with the applied potential. Inenergizing one of the row electrodes 110 a or 110 b, the correspondingrow anchor 128 a or 128 b and the corresponding row actuator beam 136 aor 136 b also becomes energized. If the resulting potential differencebetween a row actuator beam 136 a or 136 b and its opposing shutteractuator beam 135 exceeds a predetermined switching threshold, the rowactuator beam 136 a or 136 b attracts its opposing shutter actuator beam135, thereby changing the state of the shutter assembly 102.

As the actuator beams 135 and 136 are pulled together, they bend orchange shape. Each pair of actuator beams 135 and 136 (i.e., a rowactuator beam 134 a or 134 b and its opposing shutter actuator beam 135)can have one of two alternate and stable forms of curvature, eitherdrawn together with parallel shapes or curvature, or held apart in astable fashion with opposite signs to their of curvature. Thus, eachpair is mechanically bi-stable. Each pair of actuator beams 135 and 136is stable in two positions, one with the shutter 112 in an “open”position, and a second with the shutter 112 in a “closed” position. Oncethe actuator beams 135 and 136 reach one of the stable positions, nopower and no applied voltage need be applied to the column electrode 108or either row electrode 110 a or 110 b to keep the shutter 112 in thatstable position. Voltage above a predetermined threshold needs to beapplied to move the shutter 112 out of the stable position.

While both the open and closed positions of the shutter assembly 102 areenergetically stable, one stable position may have a lower energy statethan the other stable position. In one implementation, the shutterassemblies 102 are designed such that the closed position has a lowerenergy state than the open position. A low energy reset pulse cantherefore be applied to any or all pixels in order to return the entirearray to its lowest stress state, corresponding also to an all-blackimage.

The light modulation array 100 and its component shutter assemblies 102are formed using standard micromachining techniques known in the art,including lithography; etching techniques, such as wet chemical, dry,and photoresist removal; thermal oxidation of silicon; electroplatingand electroless plating; diffusion processes, such as boron, phosphorus,arsenic, and antimony diffusion; ion implantation; film deposition, suchas evaporation (filament, electron beam, flash, and shadowing and stepcoverage), sputtering, chemical vapor deposition (CVD), epitaxy (vaporphase, liquid phase, and molecular beam), electroplating, screenprinting, and lamination. See generally Jaeger, Introduction toMicroelectronic Fabrication (Addison-Wesley Publishing Co., ReadingMass. 1988); Runyan, et al., Semiconductor Integrated Circuit ProcessingTechnology (Addison-Wesley Publishing Co., Reading Mass. 1990);Proceedings of the IEEE Micro Electro Mechanical Systems Conference1987-1998; Rai-Choudhury, ed., Handbook of Microlithography,Micromachining & Microfabrication (SPIE Optical Engineering Press,Bellingham, Wash. 1997).

More specifically, multiple layers of material (typically alternatingbetween metals and dielectrics) are deposited on top of a substrateforming a stack. After one or more layers of material are added to thestack, patterns are applied to a top most layer of the stack markingmaterial either to be removed from, or to remain on, the stack. Variousetching techniques, including wet and/or dry etches, are then applied tothe patterned stack to remove unwanted material. The etch process mayremove material from one or more layers of the stack based on thechemistry of the etch, the layers in the stack, and the amount of timethe etch is applied. The manufacturing process may include multipleiterations of layering, patterning, and etching.

The process also includes a release step. To provide freedom for partsto move in the resulting device, sacrificial material is interdisposedin the stack proximate to material that will form moving parts in thecompleted device. An etch removes much of the sacrificial material,thereby freeing the parts to move.

After release the surfaces of the moving shutter are insulated so thatcharge does not transfer between moving parts upon contact. This can beaccomplished by thermal oxidation and/or by conformal chemical vapordeposition of an insulator such as Al2O3, Cr2O3, TiO2, HfO2, V2O5,Nb2O5, Ta2O5, SiO2, or Si3N4 or by depositing similar materials usingtechniques such as atomic layer deposition. The insulated surfaces arechemically passivated to prevent problems such as stiction betweensurfaces in contact by chemical conversion processes such asfluoridation or hydrogenation of the insulated surfaces.

FIG. 2 is a cross-section of an optical cavity 200 for use in a spatiallight modulator, according to an illustrative embodiment of theinvention. The optical cavity 200 includes a front reflective surface202 and a rear reflective surface 204. The front reflective surface 202includes an array of light-transmissive regions 206 through which light208 can escape the optical cavity 200. Light 208 enters the opticalcavity 200 from one or more light sources 210. The light 206 reflectsbetween the front and rear reflective surfaces 202 and 204 until itreflects through one of the light-transmissive regions 206. Additionalreflective surfaces may be added along the sides of the optical cavity200.

The front and rear reflective surfaces 202 and 204, in oneimplementation, are formed by depositing a metal or semiconductor ontoeither a glass or plastic substrate. In other implementations, thereflective surfaces 202 and 204 are formed by depositing metal orsemiconductor on top of a dielectric film that is deposited as one of aseries of thin films built-up on a substrate. The reflective surfaces202 and 204 have reflectivities above about 50%. For example, thereflective surfaces 202 and 204 may have reflectivities of 70%, 85%,96%, or higher.

Smoother substrates and finer grained metals yield higherreflectivities. Smooth surfaces may be obtained by polishing a glasssubstrate or by molding plastic into smooth-walled forms. Alternatively,glass or plastic can be cast such that a smooth surface is formed by thesettling of a liquid/air interface. Fine grained metal films withoutinclusions can be formed by a number of vapor deposition techniquesincluding sputtering, evaporation, ion plating, laser ablation, orchemical vapor deposition. Metals that are effective for this reflectiveapplication include, without limitation, Al, Cr, Au, Ag, Cu, Ni, Ta, Ti,Nd, Nb, Si, Mo and/or alloys thereof.

Alternatively, the reflective surface can be formed by interposing adielectric material of low refractive index between a light guide in theoptical cavity 200 and any of a series of thin films deposited on top ofit. The change in refractive index between the light guide and the thinfilm leads to a condition of total internal reflection within the lightguide, whereby incident light of sufficiently low incidence angle can bereflected with nearly 100% efficiency.

In the alternative, the reflective surfaces 202 or 204 can be formedfrom a mirror, such as a dielectric mirror. A dielectric mirror isfabricated as a stack of dielectric thin films which alternate betweenmaterials of high and low refractive index. A portion of the incidentlight is reflected from each interface where the refractive indexchanges. By controlling the thickness of the dielectric layers to somefixed fraction or multiple of the wavelength and by adding reflectionsfrom multiple parallel interfaces, it is possible to produce a netreflective surface having a reflectivity exceeding 98%. Some dielectricmirrors have reflectivities greater than 99.8%. Dielectric mirrors canbe custom-designed to accept a pre-specified range of wavelengths in thevisible range and to accept a pre-specified range of incident angles.Reflectivities in excess of 99% under these conditions are possible aslong as the fabricator is able to control the smoothness in thedielectric film stacks. The stacks can include between about 20 andabout 500 films.

In another alternative, the first and second reflective surfaces 202 or204 are included in the optical cavity 200 as separate components. Athin sheet of polished stainless steel or aluminum can suffice for thispurpose. Also, it is possible to produce a reflective metal surface or adielectric mirror on the surface of a continuous sheet or roll ofplastic. The sheet of reflective plastic can then be attached or adheredto other components in the optical cavity 200.

The light-transmissive regions 206 are arranged in an array to formpixels from which an image is formed. In the illustrative embodiment,the light-transmissive regions 206 are spaced between about 100 andabout 350 microns apart. The light transmissive regions are oblong orrectangular in shape, wherein the greater dimension is between about 50and about 300 microns while the narrower dimension is between 2 and 100microns, though other shapes and sizes may be suitable. For projectiondisplays the pitch can be as small as 20 microns, with aperture widthsas small as 5 microns. The ratio between the area of the frontreflective surface 202 taken up by light-transmissive regions 206 andthe total area of the front reflective surface 202 is referred to hereinas the transmissiveness ratio. Illustrative implementations of theoptical cavity 200 have transmissiveness ratios of between about 5% andabout 50%. Normally, spatial light modulators having such lowtransmissiveness ratios would emit insufficient light to form a usableimage. To ensure greater light 208 emission from the optical cavity 200,the front and rear reflective surfaces 202 and 204 reflect the light 208back and forth a number of times until the reflected light 208 passesthrough a light-transmissive region 206, or until the light 208 losesits energy from the reflections. Higher reflectivity surfaces result inmore light 208 escaping from the optical cavity 200 to form an image.Table 1, below, lists the percentage of light 208 introduced into theoptical cavity 200 that escapes through the light-transmissive regions206 (in terms of efficiency) for several transmissivenessratio/reflectivity pairings.

TABLE 1 Transmissiveness Ratio Reflectivity Efficiency  8% 0.97 59% 0.9340% 0.88 30% 14% 0.97 71% 0.93 55% 0.88 43% 20% 0.97 79% 0.93 65% 0.8853%

When the optical cavity 200 is used to form the basis of a transmissivedisplay, one or more light sources 210 introduce light into the opticalcavity 200. The light source(s) 210 may be of any suitable type,including, for example, any of the types disclosed in U.S. Pat. Nos.4,897,771 and 5,005,108, the entire disclosures of which areincorporated herein by reference. In particular, the light source(s) 210may be an arc lamp, an incandescent bulb which also may be colored,filtered or painted, a lens end bulb, a line light, a halogen lamp, alight emitting diode (LED), a chip from an LED, a neon bulb, afluorescent tube, a fiber optic light pipe transmitting from a remotesource, a laser or laser diode, or any other suitable light source.Additionally, the light sources may be a multiple colored LED, or acombination of multiple colored radiation sources 210 in order toprovide a desired colored or white light output distribution. Forexample, a plurality of colored lights such as LEDs of different colors(red, blue, green) or a single LED with multiple colored chips may beemployed to create white light or any other colored light outputdistribution by varying the intensities of each individual coloredlight. A reflector may be positioned proximate to the light source 210to reflect light 208 emitted away from the optical cavity 200 towardsthe optical cavity 200. In one implementation, three light sources 210,one red light source 210, one green light source 210, and one blue lightsource 210, sequentially introduce light 208 into the optical cavity200, alternating at frequencies in the range of 20 to 600 Hz. A rate inexcess of 100 Hz is generally faster than what the human eye can detect,thus providing a color image.

FIG. 3A is a linear cross-sectional view of a shutter assembly 300 in anopen position. The shutter assembly 300 is formed on transparentsubstrate 302 having a thickness of from about 0.3 mm to about 2 mm. Thesubstrate 302 can be, for example, made of a glass or a plastic.Suitable glasses include borosilicate glasses, or other glasses that canwithstand processing temperatures up to or exceeding 400 degreesCentigrade. Suitable plastics for the substrate 302 include, forexample, polyethyleneterephthalate (PET), or polytetrafluoroethylene(PETF), or other substantially transparent plastics that can withstandprocessing temperatures in excess of 200° C. Other candidate substratematerials include quartz and sapphire, which are understood to withstandprocessing temperatures in excess of 800° C.

The lowest layer, referred to as the “column metal layer” 304, of theshutter assembly 300 serves as the front reflective surface 202 of theoptical cavity of FIG. 2. During the process of manufacturing theshutter assembly 300, an aperture 306 is etched through the column metallayer 304 to form a light-transmissive region, such as the lighttransmissive regions 206 of FIG. 2. The aperture 306 can be generallycircular, elliptical, polygonal, serpentine, or irregular in shape. Theaperture occupies about 5% to about 25% of the area dedicated to theparticular shutter assembly 300 in the light modulation array. Otherthan at the aperture 306, the column metal layer 304 is substantiallyunbroken. The aperture 306 is filled with a dielectric material 307.Example dielectrics suitable for inclusion in the shutter assembly 300include SiO₂, Si₃N₄, and Al₂O₃.

The next layer is composed mostly of a dielectric material 307,separating the column metal layer 304 from the row electrodes 308 a and308 b disposed a layer above. The dielectric layers 316 may be between0.3 and 10 microns thick. The top layer of the shutter assembly 300includes a shutter anchor 312, two row anchors 313, two actuators, and ashutter 310. The beams of the actuators are not shown as the crosssection of the shutter assembly 300 is taken at a position in which therow actuator beams meet the row anchors 313 and the shutter actuatorbeams meet the shutter 310 (see, for example, line B-B′ on FIG. 1D). Thetop layer is supported above the lower layers by the anchors 312 so thatthe shutter 310 is free to move.

In alternative implementations, the row electrodes 308 a and 308 b arelocated at a lower layer in the shutter assembly 300 than the columnmetal layer 304. In another implementation the shutter 310 and actuatorscan be located at a layer below either of the column metal layer 304 orthe row electrodes 308 a and 308 b.

As described in relation to FIG. 1B, the actuators included in theshutter assembly may be designed to be mechanically bi-stable.Alternatively, the actuators can be designed to have only one stableposition. That is, absent the application of some form of actuationforce, such actuators return to a predetermined position, either open orclosed. In such implementations, the shutter assembly 300 includes asingle row electrode 308, which, when energized, causes the actuator topush or pull the shutter 310 out of its stable position.

FIG. 3B is a cross-sectional view of a second alternative shutterassembly 300′ in an open position according to an illustrativeembodiment of the invention. The second shutter assembly 300′ includes asubstrate 302′, a column metal layer 304′, an aperture 306′, rowelectrodes 308 a′ and 308 b′, a shutter 310′, two actuators, a shutteranchor 312′, and two row anchors 313′. The beams of the actuators arenot shown as the cross section of the shutter assembly 300′ is taken ata position in which the row actuator beams meet the row anchors 313′ andthe shutter actuator beams meet the shutter 310′. (See, for example,line B-B′ on FIG. 1D).

In the shutter assembly 300′, additional gaps are etched into the columnmetal layer 304′. The gaps electrically separate different portions ofthe column metal layer 304′ such that different voltages can be appliedto each portion. For instance, in order to reduce parasitic capacitancesthat can arise between the column metal layer 304′ and the rowelectrodes 308 a′ and 308 b′ resulting from their overlap, a voltage canbe selectively applied to the sections 314 of the column metal layer304′ that immediately underlies the row electrodes 308 a′ and 308 b′ andthe anchor 312′.

FIG. 3C is a cross-sectional view of another third alternative shutterassembly 300″ according to an illustrative embodiment of the invention.The shutter assembly 300″ includes a substrate 302″, a column metallayer 304″, an aperture 306″, row electrodes 308 a″ and 308 b″, ashutter 310″, two actuators, a shutter anchor 312″, and two row anchors313″. The beams of the actuators are not shown as the cross section ofthe shutter assembly 300″ is taken at a position in which the rowactuator beams meet the row anchors 313″ and the shutter actuator beamsmeet the shutter 310″. (See, for example, line B-B′ on FIG. 1D). Theshutter assembly 300″ includes a reflective film 316 deposited on thesubstrate 302″. The reflective film 316 serves as a front reflectivesurface for an optical cavity incorporating the shutter assembly 300″.With the exception of an aperture 306″ formed in the reflective film 316to provide a light transmissive region, the reflective film 316 issubstantially unbroken. A dielectric layer 318 separates the reflectivefilm 316 from the column metal layer 304″. At least one additionaldielectric layer 318 separates the column metal layer 304″ from the tworow electrodes 308 a″ and 308 b″. During the process of themanufacturing of the third alternative shutter assembly 300″, the columnmetal layer 304″ is etched to remove metal located below the rowelectrodes 308 a″ and 308 b″ to reduce potential capacitances that canform between the row electrodes 308 a″ and 308 b″ and the column metallayer 304″. Gaps 320 formed in the column metal layer 304″ are filled inwith a dielectric.

FIG. 3D is a cross-sectional view of a further alternative shutterassembly 300′″ in a closed position according to an illustrativeembodiment of the invention. The fourth alternative shutter assembly300′″ includes a substrate 302′″, a column metal layer 304′″, anaperture 306′″, row electrodes 308 a′″ and 308 b′″, a shutter 310′″, twoactuators, a shutter anchors 312′″, and two row anchors 313′″. The beamsof the actuators are not shown as the cross section of the shutterassembly 300′″ is taken at a position in which the row actuator beamsmeet the row anchors 313′″ and the shutter actuator beams meet theshutter 310′″. (See, for example, line B-B′ on FIG. 1D). In contrast tothe previously depicted shutter assemblies 102, 300, 300′, and 300″,much of the dielectric material used in building the fourth alternativeshutter assembly 300′″ is removed by one or more etching steps.

The space previously occupied by the dielectric material can be filledwith a lubricant to reduce friction and prevent stiction between themoving parts of the shutter assembly 300′″. The lubricant fluid isengineered with viscosities preferably below about 10 centipoise andwith relative dielectric constant preferably above about 2.0, anddielectric breakdown strengths above about 10⁴ V/cm. Such mechanical andelectrical properties are effective at reducing the voltage necessaryfor moving the shutter between open and closed positions. In oneimplementation, the lubricant preferably has a low refractive index,preferably less than about 1.5. In another implementation the lubricanthas a refractive index that matches that of the substrate 302. Suitablelubricants include, without limitation, de-ionized water, methanol,ethanol, silicone oils, fluorinated silicone oils, dimethylsiloxane,polydimethylsiloxane, hexamethyldisiloxane, and diethylbenzene.

FIG. 4 is a cross sectional view of a shutter assembly 400 with a coatedshutter 402, according to an illustrative embodiment of the invention.The shutter assembly 400 is depicted as having the general structure ofthe shutter assembly 300 of FIG. 3A. However, the shutter assembly 400can take the form of any of the shutter assemblies 102, 300, 300′, 300″,or 300′″ described above or any other shutter assembly described below.

A reflective film 404 coats the bottom of the shutter 402 to reflectlight 406 back through the shutter assembly 400 when the shutter 402 isin the closed position. Suitable reflective films 404 include, withoutlimitation, smooth depositions of Al, Cr, or Ni. The deposition of sucha film 404, if the film 404 is greater than about 0.2 microns thick,provides a reflectivity for the shutter of 95% or higher. Alternatively,amorphous or polycrystalline Si, when deposited onto a smooth dielectricsurface, can provide reflectivity high enough to be useful in thisapplication

The top of the shutter 402 is coated with a light absorbing film 408 toreduce reflection of ambient light 410 striking the top of the shutterassembly 400. The light absorbing film 408 can be formed from thedeposition and/or anodization of a number of metals, such as Cr, Ni, orAu or Si in a manner that creates a rough or porous surface.Alternatively, the light absorbing film 408 can include an acrylic orvinyl resin which includes light absorbing pigments. In alternativeimplementations of the shutter assembly 400, the absorbing film 408 isapplied to the entire, or substantially the entire top surface of theshutter assembly 400.

FIG. 5 is a cross sectional view of a shutter assembly 500 with a secondcoated shutter 502, according to an illustrative embodiment of theinvention. The shutter assembly 500 is depicted as having the generalstructure of the first alternative shutter assembly 300 of FIG. 3A.However, the shutter assembly can take the form of any of the shutterassemblies describes above 102, 300, 300′, 300″, and 300′″ or any othershutter assembly described below. In the shutter assembly 500, both thetop and the bottom of the shutter 502 are coated with a light absorbingfilm 504 such as a light absorbing film 408. The light absorbing film504 on the bottom of the shutter 502 absorbs light impacting the shutter502 in a closed position. For an optical cavity, such as optical cavity200 of FIG. 2, including the shutter assembly 500, the intensity oflight exiting the optical cavity is independent of the image beingformed. That is, light intensity is independent of the fraction ofshutters that may be in the open or the closed position.

FIG. 6 is cross-sectional view of an elastically actuated shutterassembly 600 for use in a light modulation array, such as lightmodulation array 102, according to an illustrative embodiment of theinvention. The elastically actuated shutter assembly 600 includes ametal column layer 602, a single row electrode 604, an elastic element606, and a shutter 608. The elastic element 606 provides a restoringforce which keeps the shutter 608 in an open position, away from acorresponding aperture 610 in the column metal layer 602. In the openposition, light 612 can pass through the aperture 610. Provision of aswitching voltage to the single row electrode 604 counters the force ofthe elastic element 606, thereby putting the shutter 608 into a closedposition over the aperture 610. In the closed position, the shutter 608blocks light 612 from exiting through the aperture 610. In analternative implementation, the shutter assembly 600 may include a latchto lock the shutter 608 into a closed position such that after theshutter 608 closes, the row electrode 604 can be de-energized withoutthe shutter 608 opening. To open the shutter 608, the latch is released.In still another implementation of the shutter assembly 600, the elasticactuator tends to keep the shutter 608 in a closed position. Applying avoltage to the row electrode 604 moves the shutter 608 into an openposition. Suitable spring-like elastic actuators for displays have beendescribed in U.S. Pat. No. 5,062,689, the entirety of which isincorporated herein by reference.

FIG. 7 is a cross-sectional view of a shutter assembly 700 with adeformable shutter 701 for use in a light modulation array, according toan illustrative embodiment of the invention. The shutter assembly 700includes a column metal layer 702, and one row electrode 704 formed on asubstrate 708. The deforming shutter 701, instead of translating fromone side of the shutter assembly 700 to the other side of the shutterassembly 700 to open and close, deforms in response to the energizing ofthe row electrode 704. The deforming shutter 701 is formed such that thedeforming shutter 701 retains residual stress, resulting in thedeforming shutter 701 tending to curl up out of the plane of the lightmodulation array in which it is included. By imposing a switchingvoltage between the row electrode 704 and the column metal layer 702,the deforming shutter 701 is attracted towards the substrate 708,thereby covering an aperture 710 formed in the column metal layer 702.Deformable or hinge type actuators have been described in the art, forinstance in U.S. Pat. Nos. 4,564,836 and 6,731,492, the entireties ofwhich are incorporated herein by reference.

FIG. 8A is a cross-sectional view of a shutter assembly 800 with anopaque substrate 802, such as silicon, for use in a light modulationarray, according to an illustrative embodiment of the invention. Theopaque substrate 802 has a thickness in the range of about 200 micronsto about 1 mm. Though the shutter assembly 800 resembles the shutterassembly 300 of FIG. 3A, the shutter assembly 800 can take substantiallythe same form of any of the shutter assemblies 300, 300′, 300″, 300′″,400, 500, 600, or 700 described in FIGS. 3-7. An aperture 804 is etchedthrough the entirety of the opaque substrate 802. In one implementation,the aperture 804 is formed using an anisotropic dry etch such as in aCFCl₃ gas with plasma or ion assist. The shutter assembly 800 may alsoinclude a reflective coating 810 deposited on the side of the opaquesubstrate 802 opposite the column metal layer.

FIG. 8B is a cross-sectional view of a second shutter assembly 800′ withan opaque substrate 802′ for use in a light modulation array, accordingto an illustrative embodiment of the invention. In comparison to theshutter assembly 800 in FIG. 8A, the underside of the opaque substrate800′ is etched away forming cavities 806 beneath the apertures 804′ ofthe shutter assembly 800′. The cavities 806 allow light from a largerrange of angles to escape through the aperture 804′. The larger rangeprovides for a brighter image and a larger viewing angle.

The shutter assemblies described in FIGS. 1 and 3-8 depend onelectrostatic forces for actuation. A number of alternative actuatorforcing mechanisms can be designed into shutter assemblies, includingwithout limitation the use of electromagnetic actuators, thermoelasticactuators, piezoelectric actuators, and electrostiction actuators. Othershutter motions which can be used to controllably obstruct an apertureinclude without limitation sliding, rotating, bending, pivoting,hinging, or flapping; all motions which are either within the plane ofthe reflective surface or transverse to that plane.

FIG. 9 is a cross-sectional view of a liquid crystal-based spatial lightmodulator 900. The liquid crystal-based spatial light modulator 900includes an array 901 of liquid crystal cells 902. The liquid crystalcells 902 include pairs of opposing transparent electrodes 904 on eitherside of a layer of liquid crystal molecules 906. On one side of theliquid crystal array 901, the liquid crystal-based spatial lightmodulator 900 includes a polarizer 908. On the opposite side of thearray 901, the liquid crystal-based spatial light modulator 900 includesan analyzer 910. Thus, without intervention, light passing through thepolarizer 908 would be filtered blocked by the analyzer 910. When avoltage is imposed between the transparent electrodes 904, the liquidcrystal molecules 906 between the electrodes 904 align themselves withthe resultant electric field reorienting the light passing through thepolarizer 908 such that it can pass through the analyzer 910. Thepolarizer 908 is positioned on top of a front reflective surface 911,which defines a plurality of light-transmission regions 913. The array901 is attached to an optical cavity, such as optical cavity 200 andincludes a cover plate 912. Cover plates are described in further detailin relation to FIG. 11.

Each liquid crystal cell 902 may have a corresponding red, green, orblue color specific filter. Alternatively, color differentiation can beprovided by multiple lamps operating in sequence as described above inrelation to FIG. 2.

Most liquid crystal displays (LCDs) are designed with resolutions of 80to 110 dots per inch, wherein pixel widths are in the range of 250 to330 microns. For such an LCD display, even with active matrix orthin-film transistor (TFT) addressing or switching, the transmissivenessratio of the liquid-crystal display is in the range of 75 to 90%. Forhigh-resolution applications (e.g. for document displays or projectiondisplays) in which the desired image resolution is 300 to 500 dots perinch, however, and where pixels are only 50 microns in diameter, theoverhead required for TFT addressing can limit the availabletransmissiveness ratio to about 30 or 50%. Such high-resolutiondisplays, therefore, typically suffer from a lower luminous efficiencythan their lower-resolution counterparts due to a loss of apertureratio. By constructing the liquid crystal display using an opticalcavity as described above, greater luminous efficiency can be achievedeven in high-definition LCD displays.

FIG. 10 is a cross sectional view of a first shutter-based spatial lightmodulator 1000 according to an illustrative embodiment of the invention.The shutter-based spatial light modulator 1000 includes a lightmodulation array 1002, an optical cavity 1004, and a light source 1006.The light modulation array 1002 can include any of the shutterassemblies 300, 300′, 300″, 300′″, 400, 500, 600, 700, 800, or 800′described above in FIGS. 3-8. The optical cavity 1004, in the firstshutter-based spatial light modulator 1000, is formed from a light guide1008 having front and rear surfaces. A front reflective surface 1010 isdeposited directly on the front surface of the light guide 1008 and asecond reflective surface 1012 is deposited directly on the rear surfaceof the light guide 1008.

The light guide 1008 can be formed from glass or a transparent plasticsuch as polycarbonate or polyethylene. The light guide 1008 is about 300microns to about 2 mm thick. The light guide 1008 distributes light 1014introduced into the optical cavity 1004 substantially uniformly acrossthe surface of the front reflective surface 1010. The light guide 1008achieves such distribution by means of a set of total internalreflections as well as by the judicial placement of light scatteringelements 1016. The light scattering elements 1016 can be formed in or onthe rear side of the light guide 1018 to aid in redirecting light 1014out of the light guide 1008 and through light-transmissive regions 1019formed in the front reflective surface 1010.

FIG. 11 is a cross sectional view of a second shutter-based spatiallight modulator 1100, according to the illustrative embodiment of theinvention. As with the first shutter-based spatial light modulator 1000in FIG. 10, the second shutter-based spatial light modulator 1100includes a light modulation array 1102, an optical cavity 1104, and alight source 1106. In addition, the second spatial light modulatorincludes a cover plate 1108.

The cover plate 1108 serves several functions, including protecting thelight modulation array 1102 from mechanical and environmental damage.The cover plate 1108 is a thin transparent plastic, such aspolycarbonate, or a glass sheet. The cover plate can be coated andpatterned with a light absorbing material, also referred to as a blackmatrix 1110. The black matrix can be deposited onto the cover plate as athick film acrylic or vinyl resin that contains light absorbingpigments.

The black matrix 1110 absorbs substantially all incident ambient light1112—ambient light is light that originates from outside the spatiallight modulator 1100, from the vicinity of the viewer—except inpatterned light-transmissive regions 1114 positioned substantiallyproximate to light-transmissive regions 1116 formed in the opticalcavity 1104. The black matrix 1110 thereby increases the contrast of animage formed by the spatial light modulator 1100. The black matrix 1110can also function to absorb light escaping the optical cavity 1104 thatmay be emitted, in a leaky or time-continuous fashion.

In one implementation, color filters, for example, in the form ofacrylic or vinyl resins are deposited on the cover plate 1108. Thefilters may be deposited in a fashion similar to that used to form theblack matrix 1110, but instead, the filters are patterned over the openapertures light transmissive regions 1116 of the optical cavity 1104.The resins can be doped alternately with red, green, or blue pigments.

The spacing between the light modulation array 1102 and the cover plate1108 is less than 100 microns, and may be as little as 10 microns orless. The light modulation array 1102 and the cover plate 1108preferably do not touch, except, in some cases, at predetermined points,as this may interfere with the operation of the light modulation array1102. The spacing can be maintained by means of lithographically definedspacers or posts, 2 to 20 microns tall, which are placed in between theindividual right modulators in the light modulators array 1102, or thespacing can be maintained by a sheet metal spacer inserted around theedges of the combined device.

FIG. 12A is a cross sectional view of a third shutter-based spatiallight modulator 1200, according to an illustrative embodiment of theinvention. The third shutter-based spatial light modulator 1200 includesan optical cavity 1202, a light source 1204, and a light modulationarray 1206. In addition, the third shutter-based spatial light modulator1204 includes a cover plate 1207, such as the cover plate 1108 describedin relation to FIG. 11.

The optical cavity 1202, in the third shutter-based spatial lightmodulator 1200, includes a light guide 1208 and the rear-facing portionof the light modulation array 1206. The light modulation array 1206 isformed on its own substrate 1210. Both the light guide 1208 and thesubstrate 1210 each have front and rear sides. The light modulationarray 1206 is formed on the front side of the substrate 1210. Afront-facing, rear-reflective surface 1212, in the form of a secondmetal layer, is deposited on the rear side of the light guide 1208 toform the second reflective surface of the optical cavity 1202.Alternatively, the optical cavity 1202 includes a third surface locatedbehind and substantially facing the rear side of the light guide 1208.In such implementations, the front-facing, rear-reflective surface 1212is deposited on the third surface facing the front of the spatial lightmodulator 1200, instead of directly on the rear side of the light guide1208. The light guide 1208 includes a plurality of light scatteringelements 1209, such as the light scattering elements 1016 described inrelation to FIG. 10. As in FIG. 10, the light scattering elements aredistributed in a predetermined pattern on the rear-facing side of thelight guide 1208 to create a more uniform distribution of lightthroughout the optical cavity.

In one implementation, the light guide 1208 and the substrate 1210 areheld in intimate contact with one another. They are preferably formed ofmaterials having similar refractive indices so that reflections areavoided at their interface. In another implementation small standoffs orspacer materials keep the light guide 1208 and the substrate 1210 apredetermined distance apart, thereby optically de-coupling the lightguide 1208 and substrate 1210 from each other. The spacing apart of thelight guide 1208 and the substrate 1210 results in an air gap 1213forming between the light guide 1208 and the substrate 1210. The air gappromotes total internal reflections within the light guide 1208 at itsfront-facing surface, thereby facilitating the distribution of light1214 within the light guide before one of the light scattering elements1209 causes the light 1214 to be directed toward the light modulatorarray 1206 shutter assembly. Alternatively, the gap between the lightguide 1208 and the substrate 1210 can be filled by a vacuum, one or moreselected gasses, or a liquid.

FIG. 12B is a cross sectional view of a fourth shutter-based spatiallight modulator 1200′, according to an illustrative embodiment of theinvention. As with the spatial light modulator 1200 of FIG. 12A, thefourth spatial light modulator 1200′ includes an optical cavity 1202′, alight source 1204′, a light modulation array 1206′, and a cover plate1207′, such as the cover plate 1108 described in relation to FIG. 11.The optical cavity 1202′ includes a rear-facing reflective surface inthe light modulation array 1206′, a light guide 1208′, and afront-facing rear-reflective surface 1212′. As with the third spatiallight modulator 1200, the light modulation array 1206′ of the fourthspatial light modulator 1200′ is formed on a substrate 1210′, which isseparate from the light guide 1208′.

In the fourth spatial light modulator 1200′, the light guide 1208′ andthe substrate 1210′ are separated by a light diffuser 1218 and abrightness enhancing film 1220. The diffuser 1218 helps to randomize theoptical angles of scattered light 1214′ to improve uniformity and reducethe formation of ghost images from the light source 1204 or the lightmodulation array 1206. In one implementation, the brightness enhancementfilm 1220 includes an array of optical prisms that are molded into athin plastic sheet, and which act to funnel light into a narrow cone ofillumination. The brightness enhancing film 1220 re-directs lightleaving the light guide 1208′ through light-transmissive regions 1222 atan oblique angle towards the viewer, thus resulting in an apparentincreases in brightness along the optical axis for the same input power.

FIG. 12C is a cross sectional view of a fifth shutter-based spatiallight modulator 1200″, according to an illustrative embodiment of theinvention. As with the spatial light modulator 1200 of FIG. 12A, thefifth spatial light modulator 1200″ includes an optical cavity 1202″, alight source 1204″, a light modulation array 1206″, and a cover plate1207″, such as the cover plate 1108 described in relation to FIG. 11.The optical cavity 1202″ includes a rear-facing reflective surface inthe light modulation array 1206″, a light guide 1208″, and afront-facing rear-reflective surface 1212″. As with the third spatiallight modulator 1200, the light modulation array 1206″ of the fifthspatial light modulator 1200″ is formed on a substrate 1210″, which isseparate from the light guide 1208″.

In the fifth spatial light modulator 1200″, the light guide 1208″ andthe substrate 1210″ are separated by a microlens array 1224. Themicrolens array 1224 re-directs light 1214″ leaving the light guide1208″ through light-transmissive regions 1222′ at an oblique angletowards the viewer, thus resulting in an apparent increases inbrightness for the same input power.

In addition, since the light modulation array 1206″ in the fifthshutter-based spatial light modulator 1200″ is formed on its ownsubstrate 1210″, separate from the light guide 1208″, the light guide1208″ can be constructed of a moldable plastic, without the transitiontemperature of the plastic limiting the manufacturing processesavailable for constructing the light modulation array 1210″. Thus, thelight guide 1208″ can be molded to substantially encapsulate the lightsource 1204″ used to introduce light 1214″ into the optical cavity1202″. The encapsulation of the light source 1204″ into the light guide1208″ provides improved coupling of light 1214″ into the light guide1208″. Similarly, scattering elements 1209″ can be incorporated directlyin the mold for the light guide 1208″.

FIG. 12D is a cross-sectional view of a sixth illustrative embodiment ofa shutter-based light modulation array 1200′″. As with the spatial lightmodulator 1200 of FIG. 12A, the sixth spatial light modulator 1200′″includes an optical cavity 1202′″, a light source 1204′″, a lightmodulation array 1206′″, and a cover plate 1207′″, such as the coverplate 1108 described in relation to FIG. 11. The optical cavity 1202′″includes a rear-facing reflective surface in the light modulation array1206′″, a light guide 1208′″, a front-facing rear-reflective surface1212′″, a diffuser 1218′″, and a brightness enhancing film 1220′″.

The space between the light modulation array 1206′″ and the cover plate1207′″ is filled with a lubricant 1224, such as the lubricant describedin relation to FIG. 3D. The cover plate 1207′″ is attached to theshutter assembly 1206 with an epoxy 1225. The epoxy should have a curingtemperature preferably below about 20° C., it should have a coefficientof thermal expansion preferably below about 50 ppm per degree C. andshould be moisture resistant. An exemplary epoxy is EPO-TEK B9021-1,sold by Epoxy Technology, Inc. The epoxy also serves to seal in thelubricant 1224.

A sheet metal or molded plastic assembly bracket 1226 holds the coverplate 1207′″, the light modulation array 1206′″, and the optical cavity1202′″ together around the edges. The assembly bracket 1226 is fastenedwith screws or indent tabs to add rigidity to the combined device. Insome implementations, the light source 1204′″ is molded in place by anepoxy potting compound.

FIG. 13 is a cross-sectional view of a seventh shutter-based spatiallight modulator 1300 according to an illustrative embodiment of theinvention. The seventh shutter-based spatial light modulator 1300includes a substrate 1302 on which a light modulation array 1304 isformed, and a light guide 1306. The light modulation array 1304 includesa front reflective surface for the optical cavity 1310 of the spatiallight modulator 1300. A reflective material is deposited or adhered tothe rear side of the light guide to serve as a rear reflective surface1308. The rear side of the light guide 1306 is angled or shaped withrespect to the front side of the light guide 1308 to promote uniformdistribution of light in the light modulation array 1304. The rearreflective surface 1308, however, is still partially facing the frontreflective surface.

FIG. 14A is a cross-sectional view of another spatial light modulator1400, according to an illustrative embodiment of the invention. Thespatial light modulator 1400 includes a substrate 1402 on which a lightmodulation array 1404 is formed. The light modulation array includes areflective surface serving as a front reflective surface 1405 of anoptical cavity. The spatial light modulation 1400 also includes a rearreflective surface 1406 substantially facing the rear side of the lightmodulation array 1404. A light source 1408 is positioned within thespace formed between the substrate 1402 on which the light modulationarray 1404 is formed and the rear reflective surface 1406. The space mayalso be filled with a substantially transparent plastic into which thelight source 1408 is embedded.

FIG. 14B is a cross-sectional view of another spatial light modulator1400′, similar to the spatial light modulator 1400 of FIG. 14A. Thespatial light modulator 1400′ includes a substrate 1402′ on which alight modulation array 1404′ is formed. The light modulation array 1404′includes a reflective surface serving as a front reflective surface 1405of an optical cavity. The spatial light modulation 1400′ also includes arear reflective surface 1406′. The rear reflective surface 1406′ iscorrugated, textured, or shaped to promote light distribution in theoptical cavity formed by the reflective surfaces (i.e., the rearreflective surface 1406′ and a reflective surface incorporated into thelight modulation array 1404′ of the spatial light modulator 1400′.

FIG. 15 is a cross-sectional view of another shutter assembly 1500 foruse in a light modulation array, according to an illustrative embodimentof the invention. The shutter assembly 1500 includes a metal columnlayer 1502, two row electrodes 1504 a and 1504 b, a shutter 1506, builton a substrate 1509. The shutter assembly 1500 also includes one or morelight scattering elements 1508. As with other implementations of theshutter assemblies described above, an aperture 1510 is etched throughthe column metal layer 1502. The light scattering elements 1510 caninclude any change in the shape or geometry of the substrate 1509, suchas by roughening, coating, or treating the surface of the substrate1509. For example, the light scattering elements can include patternedremnants of the column metal 1502 having dimensions of about 1 to about5 microns. The light scattering elements 1508 aid in extracting light1512 trapped in the substrate 1508 due to total internal reflection.When such trapped light 1512 strikes one of the scattering elements1508, the angle of the light's 1512 path changes. If the angle of thelight's 1512 path becomes sufficiently acute, it passes out of thesubstrate 1509. If the shutter 1506 is in the open position, thescattered light 1512 can exit the aperture 1510, and proceed to a vieweras part of an image.

FIG. 16 is a cross sectional view of yet another spatial light modulator1600 according to an illustrative embodiment of the invention. Thespatial light modulator 1600 includes a light modulation array 1602formed on the rear surface of a substrate 1604, facing the interior ofan optical cavity 1606. The individual light modulation elements 1608,such as the shutter assemblies 300, 300′, 300″, 300′″, 400, 500, 600,700, 800, and 800′ described in FIGS. 3-8 or the liquid-crystal cells902 described in FIG. 9, making up the light modulation array 1602 aremodified to reverse the sides of the light modulation elements 1608 thatreflect or absorb light as compared to what is described with referenceto FIGS. 4 and 5.

The optical cavity 1606 includes both a front reflective surface 1610, arear reflective surface 1612, and a light guide 1614. Light isintroduced into the optical cavity by a light source 1613. The frontreflective surface 1610 is disposed on front-facing surface of the lightguide 1614, providing a substantially continuous layer of highreflectivity and also defining light transmissive region 1616. The frontreflective surface 1610 is separated from the light modulation array1602 by a transparent gap 1618. The gap 1618 is preferably narrower thanwidth of the light transmissive regions 1616, less than, for example,about 100 microns. The gap 1618 may be as narrow as about 10 micronswide, or even narrower.

In one implementation, the gap 1618 is filled with a lubricant 1620,such as the lubricant described in relation to FIG. 3D. The lubricant1620 may have a refractive index that substantially matches that of thelight guide 1614 to facilitate the extraction of light from the lightguide 1614.

The spatial light modulator 1600 can optionally forego a cover plate,since the shutter assembly is protected by the environment by thesubstrate 1604. If a cover plate is omitted, a black matrix, such as theblack matrix 1110 of FIG. 11, can be applied to the front-facing surfaceof the substrate 1604.

FIG. 17 is a cross-sectional view of a transflective shutter assembly1700, according to an illustrative embodiment of the invention, whichcan be incorporated into the spatial light modulators 1000, 1100, 1200,1300, 1400, and 1500 described in FIGS. 10-15. The transflective shutterassembly 1700 forms images from both light 1701 emitted by a lightsource positioned behind the shutter assembly 1700 and from ambientlight 1703. The transflective shutter assembly 1700 includes a metalcolumn layer 1702, two row electrodes 1704 a and 1704 b, and a shutter1706. The transflective shutter assembly 1700 includes an aperture 1708etched through the column metal layer 1702. Portions of the column metallayer 1702, having dimensions of from about 1 to about 5 microns, areleft on the surface of the aperture 1708 to serve as transflectionelements 1710. A light absorbing film 1712 covers the top surface of theshutter 1706.

While the shutter is in the closed position, the light absorbing film1712 absorbs ambient light 1703 impinging on the top surface of theshutter 1706. While the shutter 1706 is in the open position as depictedin FIG. 17, the transflective shutter assembly 1700 contributes to theformation of an image both by allowing light 1701 to pass through thetransflective shutter assembly originating from the dedicated lightsource and from reflected ambient light 1703. The small size of thetransflective elements 1710 results in a somewhat random pattern ofambient light 1703 reflection.

The transflective shutter assembly 1700 is covered with a cover plate1714, which includes a black matrix 1716. The black matrix absorbslight, thereby substantially preventing ambient light 1703 fromreflecting back to a viewer unless the ambient light 1703 reflects offof an uncovered aperture 1708.

FIG. 18 is a cross-sectional view of a second transflective shutterassembly 1800 according to an illustrative embodiment of the invention,which can be incorporated into the spatial light modulators 1000, 1100,1200, 1300, 1400, and 1500 described in FIGS. 10-15. The transflectiveshutter assembly 1800 includes a metal column layer 1802, two rowelectrodes 1804 a and 1804 b, and a shutter 1806. The transflectiveshutter assembly 1800 includes an aperture 1808 etched through thecolumn metal layer 1702. At least one portion of the column metal layer1802, having dimensions of from about 5 to about 20 microns, remains onthe surface of the aperture 1808 to serve as a transflection element1810. A light absorbing film 1812 covers the top surface of the shutter1806. While the shutter is in the closed position, the light absorbingfilm 1812 absorbs ambient light 1803 impinging on the top surface of theshutter 1806. While the shutter 1806 is in the open position, thetransflective element 1810 reflects a portion of ambient light 1803striking the aperture 1808 back towards a viewer. The larger dimensionsof the transflective element 1810 in comparison to the transflectiveelements 1710 yield a more specular mode of reflection, such thatambient light originating from behind the viewer is substantiallyreflected directly back to the viewer.

The transflective shutter assembly 1800 is covered with a cover plate1814, which includes a black matrix 1816. The black matrix absorbslight, thereby substantially preventing ambient light 1803 fromreflecting back to a viewer unless the ambient light 1803 reflects offof an uncovered aperture 1808.

Referring to both FIGS. 17 and 18, even with the transflective elements1710 and 1810 positioned in the apertures 1708 and 1808, some portion ofthe ambient light 1703 and 1803 passes through the apertures 1708 and1808 of the corresponding transflective shutter assemblies 1700 and1800. When the transflective shutter assemblies 1700 and 1800 areincorporated into spatial light modulators having optical cavities andlight sources, as described above, the ambient light 1703 and 1803passing through the apertures 1708 and 1808 enters the optical cavityand is recycled along with the light introduced by the light source. Inalternative transflective shutter assemblies, the apertures in thecolumn metal are at least partially filled with asemi-reflective-semitransmissive material.

FIG. 19 is a cross sectional view of a front reflective shutter assembly1900 according to an illustrative embodiment of the invention. The frontreflective shutter assembly 1900 can be used in a reflective lightmodulation array. The front reflective shutter assembly 1900 reflectsambient light 1902 towards a viewer. Thus, use of arrays of the frontreflective shutter assembly 1900 in spatial light modulators obviatesthe need for a dedicated light source in viewing environments havinghigh amounts of ambient light 1902. The front reflective shutterassembly 1900 can take substantially the same form of the shutterassemblies 300, 300′, 300″, 300′″, 400, 500, 600, 700, 800 or 800′ ofFIGS. 3-8. However, instead of the column metal layer of the shutterassemblies 300, 400, 500, 600, 700, or 800 including an aperture toallow passage of light, the column metal layer includes a reflectivesurface beneath the position of a closed shutter 1904. The front-mostlayer of the reflective shutter assembly 1900, including at least thefront surface of the shutter 1904, is coated in a light absorbing film1908. Thus, when the shutter 1904 is closed, light 1902 impinging on thereflective shutter assembly 1900 is absorbed. When the shutter 1904 isopen, at least a fraction of the light 1902 impinging on the reflectiveshutter assembly 1900 reflects off the exposed column metal layer 1910back towards a viewer. Alternately the column metal layer 1910 can becovered with an absorbing film while the front surface of shutter 1908can be covered in a reflective film. In this fashion light is reflectedback to the viewer only when the shutter is closed.

As with the other shutter assemblies and light modulators describedabove, the reflective shutter assembly 1900 can be covered with a coverplate 1910 having a black matrix 1912 applied thereto. The black matrix1912 covers portions of the cover plate 1910 not opposing the openposition of the shutter.

FIG. 20 is an isometric view of a spatial light modulator 2000 includingmultiple light modulation arrays 2002, according to an illustrativeembodiment of the invention. The size of several of the light modulationarrays 2002 described above is limited, somewhat, by the semiconductormanufacturing techniques used to construct them. However, light guides2004 and reflective films 2006 can be formed on a significantly largerscale. A spatial light modulator which includes multiple, adjacentlydisposed light modulation arrays 2002, arranged over one or more lightguides 2004, can generate a larger image, thereby circumventing theselimitations.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The forgoingembodiments are therefore to be considered in all respects illustrative,rather than limiting of the invention.

1. A display apparatus comprising: a layer of reflective material; anarray of MEMS-based light modulators supported proximate the layer ofreflective material, wherein each of the MEMS-based light modulatorsincludes a modulation element and an actuator for moving the modulationelement relative to the layer of reflective material.
 2. The displayapparatus of claim 1, comprising a plurality of apertures formed in thelayer of reflective material for allowing light to pass through thelayer of reflective material.
 3. The display apparatus of claim 2,wherein the modulation element comprises a shutter for selectivelyobstructing at least one of the plurality of apertures.
 4. The displayapparatus of claim 2, wherein the each aperture of the plurality ofapertures corresponds to one of the MEMS-based light modulators.
 5. Thedisplay apparatus of claim 2, comprising a spacer disposed between thelayer of reflective material and the array to keep the modulationelements about a predetermined distance from the layer of reflectivematerial.
 6. The display apparatus of claim 1, wherein the modulationelements of the MEMS-based light modulators are maintained less thanabout 100 microns away from the layer of reflective material.
 7. Thedisplay apparatus of claim 1, wherein the modulation elements of theMEMS-based light modulators are maintained less than about 10 micronsaway from the layer of reflective material.
 8. The display apparatus ofclaim 1 the modulation elements of the MEMS-based light modulators aremaintained less than about 10 microns away from the layer of reflectivematerial.
 9. The display apparatus of claim 1, wherein the layer ofreflective material reflects light impinging on a surface of the layerof reflective material facing away from the array of MEMS-based lightmodulators.
 10. The display apparatus of claim 9, wherein the layer ofreflective material absorbs light impinging on a surface of the layer ofreflective material facing towards from the array of MEMS-based lightmodulators.
 11. The display apparatus of claim 9, comprising a secondlayer of reflective material positioned on an opposite side of the layerof reflective material from the MEMS-based light modulators, and facingtowards the layer of reflective materials.
 12. The display apparatus ofclaim 11, comprising a light guide positioned between the layer ofreflective material and the second layer of reflective material.
 13. Thedisplay apparatus of claim 1, comprising a liquid located at leastbetween the modulation elements and the layer of reflective material.14. The display apparatus of claim 13, wherein the liquid is alubricant.
 15. The display apparatus of claim 1, wherein the layer ofreflective material comprises a dielectric mirror.
 16. The displayapparatus of claim 1, wherein the layer of reflective material comprisesa metal.
 17. The display apparatus of claim 1, wherein the MEMS-basedlight modulators are configured for moving the modulation elementssubstantially in a plane parallel to the layer of reflective material.